The "secret" part is that they include information that is only interesting to law enforcement (timestamps and durations) in addition to pulling information from third part communication tools like WhatsApp.
Namely, Apple chose to provide this information to government surveillance when they could have (and indeed promised) to design it in a way that allows for the iPhone to be used in sensitive situations and by people who need security from state coercion and violence.
This doesn't make sense to me. Law enforcement has been able to get that information easily for quite some time directly from telephony systems. Timestamps and durations have been a part of phone UIs for quite some time, which to me indicates that it is in fact interesting to more than law enforcement... unless phone manufacturers conspired to display that information to help LEOs when they confiscate phones.
Anyone who cares about state coercion and violence should know phone calls are about the least sensitive way to communicate.
> Timestamps and durations have been a part of phone UIs for quite some time
This is not familiar to me at all. If true, note the remaining points: the manner in which this data was synchronized allowed this material to be provided to law enforcement (Apple had other options) and Apple also included information from third party communication tools used by some people (mistakenly) to avoid being surveilled.
Timestamps and durations have been visible on the iPhone ever since it was released in 2007. I assume other smartphones do something similar. Integration with third parties is a more recent feature, it's what lets you use VoIP apps with the iPhone's native call interface, and those calls end up in the same call history that your "regular" phone calls do, which is why they're included in the syncing.
Namely, Apple chose to provide this information to government surveillance when they could have (and indeed promised) to design it in a way that allows for the iPhone to be used in sensitive situations and by people who need security from state coercion and violence.